1. Field of the Invention
The invention generally relates to an image forming apparatus having a recording head for ejecting ink droplets and more specifically to a drive control of the recording head of the image forming apparatus.
2. Description of the Related Art
An inkjet recording apparatus is generally known as an inkjet image forming apparatus having a liquid ejecting head (inkjet head or recording head) for ejecting ink droplets. Examples of such an inkjet image forming apparatus having the inkjet head include image forming apparatuses having a function of a printer, a facsimile machine, or a plotter, or a combination of these functions. The inkjet image forming apparatus having the inkjet head is configured to eject ink droplets from the inkjet head onto a transferred sheet (recording medium) to form an image. The formation of the image also includes recording, printing, and imaging. There are two types of the inkjet image forming apparatus including 1) a serial type image forming apparatus in which the inkjet head ejects ink droplets onto the sheet to form an image while traveling in a main-scanning direction; and 2) a line type image forming apparatus in which the inkjet head ejects ink droplets onto the sheet to form an image without traveling. Note that the sheet is not limited to paper but includes any media including an OHP insofar as ink droplets or other liquid can be adhered to the media. Such media are also referred to as a subject recording medium or a recording medium, a recording sheet, and a recording form.
Note that in this application, the “inkjet image forming apparatus” indicates an image forming apparatus that forms an image onto media such as paper, string, fiber, fabric, leather, metal, plastic, glass, wood, and ceramics by ejecting liquid onto such media. Note also that “forming an image” or “image formation” not only indicates providing an image having some kind of meanings onto the media such as characters and symbols, but also indicates an image without having any meanings such as patterns (i.e., by simply ejecting ink droplets onto the media). Further, “ink” is not limited to those generally called “ink”, but includes the name “ink” used as a generic name for liquid capable of forming an image such as recording liquid, fixing liquid, and “liquid”. The ink in this application also indicates DNA specimens, resist, patterning material, resin, and the like. Moreover, the “image” is not limited to the image applied to a two-dimensional object but includes the image applied to a three-dimensional object and the image formed of a molded object.
As described above, there are two types of the inkjet image forming apparatus including 1) the serial type image forming apparatus in which the inkjet head is attached to a carriage so that the inkjet head travels in the main-scanning direction perpendicular to a direction toward which the sheet is transferred while ejecting ink droplets onto the sheet to form an image; and 2) the line type image forming apparatus having a line type head with plural nozzle arrays from which ink droplets are ejected to an approximately entire length of a recording region of the sheet to form an image.
In such image forming apparatuses, plural drive pulses (ejecting pulses) for ejecting ink droplets within one printing cycle are generated in time series to form common drive waveforms. For example, when relatively large dots are formed, two or more drive pulses are selected so that the inkjet head ejects plural ink droplets based on the selected drive pulses. Accordingly, the plural ink droplets are, while being ejected, combined to form a dot or a droplet having various sizes. Or, non-ejecting pulses based on which the head is driven without ejecting ink droplets are also generated and the generated non-ejecting pulses are combined into the common drive waveforms. When the non-ejecting pulses are selected, the inkjet head is slightly driven or slightly oscillates.
For example, Japanese Patent No. 3671955 (also referred to as Patent Document 1) discloses an inkjet apparatus that includes a first drive signal generator unit capable of generating a first drive signal having at least two first drive pulses composed of plural waveform components within one ejecting cycle, and a second drive signal generator unit capable of generating a second drive signal having at least one second drive pulse composed of plural waveform components within one ejecting cycle, where part of the first pulse and part of the second pulse are selected to form a non-ejecting pulse.
Japanese Patent Application Publication No. 2003-118107 (also referred to as Patent Document 2) and Japanese Patent Application Publication No. 2002-154207 (also referred to as Patent Document 3) disclose an inkjet apparatus that includes a drive signal generator unit generating a drive signal for generating a micro-vibration pulse applied to a pressure generator unit to minutely oscillate plural types of ejecting pulses and a liquid meniscus, a pulse generator unit generating the minute oscillation pulses and the ejecting pulses by selecting parts of the drive pulses forming the drive signal, where a start end and a terminal end of the drive signal are at a common potential, at least one of a plurality of kinds of ejecting pulses includes a waveform component having a terminal end at a potential different from the common potential, and adjusting waveform components for connecting the terminal end preset at the potential different from the common potential to a point preset at the common potential, and waveform components which constitute parts of minute oscillation pulses are used as at least parts of the adjusting waveform components.
Japanese Patent No. 4032338 (also referred to as Patent Document 4) discloses an inkjet apparatus having a pressure generator unit capable of changing ink pressure in a chamber by expanding or contracting the chamber based on drive pulses such that ink droplets are ejected from nozzle openings based on the pressure change in the chamber. The inkjet apparatus includes a drive signal generator unit generating a first drive signal and a drive pulse generator unit generating a drive pulse based on the drive signal, where the drive pulse generator unit generates a first drive pulse including an expanding waveform component to expand the chamber and hold the expanded chamber, a first filling waveform component to further expand the chamber expanded by the expanding waveform component, and a first ejecting waveform component to contract the chamber expanded by the first filling waveform component, and the drive pulse generator unit also generates a second drive pulse including a contracting waveform component to contract the chamber and hold the contracted chamber, a second filling waveform component to expands the contracted chamber held by the contracting waveform component to supply ink in the chamber, and a second ejecting waveform component to contract the chamber expanded by the second filling waveform component, thereby, generating different gray scale pulses based on the first and second drive pulses.
Japanese Patent No. 4251912 (also referred to as Patent Document 5) discloses an inkjet apparatus having a head that is driven by selecting a desired one of driving waveforms having at least an ejecting pulse for ejecting liquid droplets, a first dummy pulse and a second dummy pulse having voltages smaller than that of the ejecting pulse before and after the liquid droplet ejection, thereby generating a non-ejecting pulse generating energy that generates a pulse width longer than the ejecting pulse from part of the first dummy pulse and part of the second dummy pulse without allowing a nozzle to eject liquid droplets.
When plural ink droplets ejected from the head are combined to form plural dots of different sizes, ink droplets having different sizes (i.e., amounts of droplets) to be ejected are determined based on pulse shapes (an element forming a pulse is called a “waveform component”) and a driving timing (i.e., a timing to drive). If an ejecting pulse capable of ejecting ink droplets of different sizes (i.e., a pulse for driving a pressure generator unit to eject ink droplets) is generated as a designated pulse and the generated ejecting pulse is embedded in a common drive waveform, the entire drive waveform is elongated. As a result, the drive frequencies are lowered and the printing speed is decreased.
In the inkjet apparatus disclosed by Patent Document 1 that is configured to utilize two drive signals (i.e., drive waveform in the present embodiments), the configuration of the two drive signals to generate and/or select drive waveforms may become complicated. In the inkjet apparatus disclosed by Patent Documents 2 and 3 that is configured to utilize part of the adjusting waveform components embedded in the ejecting pulse for part of the minute oscillation pulse, the ejecting pulse is designated and has a longer drive waveform, which makes it difficult to increase the printing speed. Further, the inkjet apparatus disclosed by Patent Document 4 is configured to generate pulses having different tones based on the first and second drive pulses. However, the inkjet apparatus is not configured to generate different pulse shapes for corresponding droplet sizes, though utilizing part of the drive pulse. Accordingly, the entire drive waveform is still elongated. As a result, it is difficult to increase the printing speed. Moreover, in the inkjet apparatus disclosed by Patent Document 5 that is configured to form the minute drive pulse, the ejecting pulse is designated, and has a longer entire drive waveform. As a result, it is difficult to increase the printing speed.